When Ed Rendell became the mayor of Philadelphia in 1992, he started a fight with the city’s labor unions that will sound familiar to anyone who has been following the recent news from Wisconsin. In his inaugural address, Mr. Rendell, a Democrat, announced, “Philadelphia stands on the brink of total disaster.”

He told the city’s unions that they needed to accept less generous health benefits, fewer holidays and a pay freeze. The unions promised to strike. Mr. Rendell pointed out, frequently and publicly, that the city offered better benefits than private companies did. The public sided with the mayor, and on most issues, the unions eventually caved.

If you review the recent history of battles between unions and state or local governments, you’ll find similar stories. In New York, Rudolph Giuliani won big concessions. In Chicago, Richard Daley did, too. In Wisconsin — setting aside Gov. Scott Walker’s attempt to end collective bargaining — unions have already agreed to a significant cut in take-home pay.

Of course, to deny that public unions are uniquely powerful is not to deny that they are powerful. And, as Leonhardt points out, public unions often resist improvements to government efficiency. Worse, the combination between the short time horizons of politicians, who want to keep taxes low, and the desire of public unions to boost compensation often results in pay being shifted into extremely generous pension and early-retirement schemes.

Still, there's plenty of evidence that the political system can push back against public unions without attacking their right to bargain collectively. And when you recognize that massive pension shortfalls are a relatively recent phenomenon stemming mostly from the financial collapse, then the mismatch between the underlying problem and Scott Walker's response becomes all the more stark.